Lighting optical machine and defect inspection system

ABSTRACT

A lighting optical machine and defect inspection system having high reliability and safety when a laser beam is used as a light source. The lighting optical machine comprises: a housing, which accommodates a laser source, a beam polarization mechanism having first and second plane mirrors enabling a beam emitted from the laser source to be reflected so that the beam travels in the direction almost parallel to the beam emitted from the laser source, a beam expander for converting the beam to a parallel beam having a larger cross-sectional area, an objective lens, through which the parallel beam is reduced and applied to the surface of a sample; a first control mechanism for controlling the directions of the two plane mirrors of the beam polarization mechanism with an electric signal; and a second control mechanism for controlling the focus position of the beam expander with an electric signal.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a lighting optical machine and a defectinspection system used for the inspection or observation of criticaldimensional pattern defects and foreign matters typically found inmanufacturing processes of semiconductor devices or flat panel displays.

2. Background Art

As semiconductors are highly integrated, circuit patterns tend to befiner than ever. Under these circumstances, higher and higher resolutionis required for detecting defects of circuit patterns on wafers, whichare lithographed through exposure from circuit patterns formed on masksor reticles for use in photolithography processes for manufacturingsemiconductors. In order to enhance the resolution, a lighting beam maybe changed from visible light to ultraviolet light so that the beam hasa shorter wavelength. Conventionally, an Hg lamp has been used as alight source, and among various emission lines generated from an Hglamp, those with required wavelengths have been optically selected foruse. However, the emission lines of the Hg lamp have a broader emissionspectrum and it is difficult to correct optical color aberrationsthereof. Further, in order to obtain sufficient illuminance, a largelight source is necessary, resulting in decreased efficiency.

In recent years, an exposure device carrying a KrF excimer laser with awavelength of 248 nm as a light source therefor in the semiconductormanufacturing processes has been developed. However, the excimer laserlight source is large and predetermined safety measures must be takendue to the use of fluorine gas.

Examples of ultraviolet laser light sources include a laser devicewherein the wavelength of a solid-state YAG laser light is convertedwith a nonlinear optical crystal and an Ar—Kr laser device, and a laserbeam with the wavelength of 266 or 355 nm can be obtained thereby. It isadvantageous that these laser devices have a larger output power incomparison with lamps conventionally used as light sources, and generatea parallel pencil, the beam passage of which can freely be routed. Onthe other hand, due to the coherence properties of lasers, laser speckleoccurs, and causing adverse influences such as uneven brightness indetecting the circuit patterns formed on a sample. Incidentally, JPPatent Publication (Kokai) No. 2001-141428 A discloses a solution tothis problem as a conventional technology. However, the technology ofthe above publication is not directed at reliability regarding thedetection accuracy, such as optical axis adjustment and luminous energyadjustment, or at safety during the use of a laser.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a lighting opticalmachine and a defect inspection system having high reliability andsafety when a laser beam is used as a light source.

According to an embodiment of the present invention, a lighting opticalmachine comprises:

a housing, wherein the housing accommodates a laser source, a beampolarization mechanism having a first and a second plane mirrors, whichenable a beam emitted from the laser source to be reflected so that thebeam travels in the direction almost parallel to the beam emitted fromthe laser source, a beam expander for converting the beam to a parallelbeam having a larger cross sectional area, and an objective lens,through which the parallel beam is reduced and applied to the surface ofa sample;

a first control mechanism for controlling the directions of the twoplane mirror of the beam polarization mechanism with an electric signal;and

a second control mechanism for controlling the focus position of thebeam expander with an electric signal,

A further embodiment of the present invention is a pattern defectinspection system provided with the above lighting optical machine.

This specification includes part or all of the contents as disclosed inthe specification and/or drawings of Japanese Patent Application No.2002-200720, which is a priority document of the present application.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a view showing the configuration of a lighting opticalmechanism according to the present invention.

FIG. 2 is a perspective view showing the configuration of a mechanicalpart of an optical wafer defect inspection.

FIG. 3 is a perspective view showing the configuration of a low coherentoptical unit 6.

DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to FIGS. 1 to 3. FIG. 1 shows one exemplary configuration of alighting optical system according to the present invention. In thepresent invention, in order to accomplish high brightness lighting in ashort wavelength area, a far ultraviolet laser beam is used as a laserbeam source 3. A laser beam L1 emitted (oscillated) from the laser beamsource 3 changes its angle at a first plane mirror 4 a and changes itsangle again at a second plane mirror 4 b so as to be almost parallel toa beam emitted from the laser beam source 3.

Next, a beam expander 5 enables the laser beam L1 to become a parallelpencil having a large cross-sectional diameter. The beam then entersinto an objective lens 11 through a low coherent optical unit, a beamsplitter for polarization, polarization devices, or the like, with whichan object to be measured is irradiated. The laser beam L1 expanded bythe beam expander 5 is converged around the pupil of the objective lens11 with a lens on the way to the objective lens 11, and thereafter usedfor Kohler's illumination on a sample.

Further, the first and second plane mirrors 4 a and 4 b are coupled to abeam polarization mechanism 40 a and 40 b, respectively, which aredriven by a motor, etc. to change the angle of the plane mirrors.Furthermore, they are connected to a beam polarization mechanism controlpart 41 for controlling their angles. Moreover, the beam polarizationmechanism control part 41 is coupled to a manual operation input part42, through which the polarization angle can be manually controlled.

In addition, the beam expander 5 is coupled to a beam expanderadjustment mechanism 50 driven by a motor, etc., and the beam can bechanged to a parallel pencil having an enlarged cross-sectional diameterby changing the focus position. Further, it is connected to a beamexpander adjustment mechanism control part 51 to control the size of thecross-sectional diameter. The beam expander adjustment mechanism controlpart 51 is connected to a manual operation input part 52, through whichthe size of cross-sectional diameter of the beam can be manuallycontrolled.

Also, a first beam splitter 30 a is provided after the beam expander 5for amplitude splitting of the parallel pencil, which is further dividedin two with a second beam splitter 30 b. One of the divided beams entersinto a beam profile observation camera 31 so that the beam shape thereofis obtained. Using the obtained shape, the beam cross-sectional diametercan be measured. Thus, when the obtained value is not that of apredetermined cross-sectional diameter, an instruction is sent to thebeam expander adjustment mechanism control part 51 to adjust thecross-sectional diameter. Based on the instruction, the beamcross-sectional diameter is automatically adjusted to achieve thepredetermined value.

The other divided beam passes through a convergence lens 33 and isconverged on a beam spot positioning sensor 32 so that a beam positiondisplacement is detected. When a position displacement is found, aninstruction to adjust the beam position to the center is sent to thebeam polarization mechanism control part 41. Based on the instruction,the beam spot position is automatically adjusted to the center. Asdescribed above, it is possible to maintain a constantly stable lightbeam.

Additionally, there may be provided a display monitor 60, which monitorsinformation from the beam profile observation camera 31 and beam spotpositioning sensor 32, or a communication means 61.

All portions except the manual operation input parts 42 and 52 areaccommodated in a housing 62, and thereby there is no fear of a laserbeam leaking outside. When the laser beam is adjusted, the position,angle, or cross-sectional diameter of the beam can be adjusted with themanual operation input parts 42 and 52 present outside the housing 62,so that there is no chance that an operator may be exposed to the laserbeam and the laser beam can be adjusted by remote control. The housing62 may have a structure such as a shape for covering all portions exceptthe manual input parts 42 and 52. It may also have a shape withsleeve-shaped members for covering beam passages and a slightly largercovering part for the lens, etc. Further, it is not necessary to conductoperations in a limited narrow space, and an operator can conductoperations through remote control, so that the labor of the operator canbe reduced. It is preferable to install the display monitor 60 orcommunication means 61 outside the housing 62.

Next, FIG. 2 shows one exemplary configuration of a lighting opticalmechanism of an optical wafer defect inspection system as an apparatusprovided with the above lighting optical machine. However, the figurepartially contains a flow chart regarding the image processingmechanism. In the present invention, a ultraviolet laser beam is used asa light source to achieve high brightness in a short wavelength area.

A stage 2 has degrees of freedom in directions of the X, Y, Z, and θaxes, and a semiconductor wafer 1 having one example pattern to beinspected is mounted as a sample. The laser beam L1 emitted from thelaser beam source 3 enters into the objective lens 11 through a mirror 4comprising a first plane mirror 4 a and a second plane mirror 4 b, abeam expander 5, a low coherent optical unit 6, a lens 7, a polarizationbeam splitter 9, and polarization devices 10. It is then applied to thesemiconductor wafer 1 as one example pattern to be inspected.

The beam expander 5 expands a laser beam to a certain size. The expandedlaser beam L1 is converged in the vicinity 11 a of the pupil of theobjective lens 11 with the lens 7, and thereafter used for Kohler'sillumination on the sample.

Further, the first plane mirror 4 a and the second plane mirror 4 b arecoupled to beam polarization mechanisms 40 a and 40 b in order to changetheir angles. The beam expander 5 is also coupled to a beam expanderadjustment mechanism 50 capable of changing its focus position.Furthermore, a first beam splitter 30 a is provided after the beamexpander 5 for amplitude-splitting of a parallel pencil, and a secondbeam splitter 30 b separates the beam in two. One of the divided beamsenters into a beam profile observation camera 31 so that the shape ofthe beam is obtained. The other divided beam passes through aconvergence lens 33 and is converged on a beam spot positioning sensor32 so that a beam position displacement is detected.

A reflecting beam from the sample is detected with an image sensor 13via the objective lens 11, the polarization devices 10, the polarizationbeam splitter 9, and an image formation lens 12, which are arrangedvertically from above the sample. The polarization beam splitter 9reflects the beam, when the polarization direction of the leaser beam isparallel to its reflecting face. When the direction is perpendicular tothe reflecting face, the splitter allows the beam to be transmittedtherethrough. The laser beam used as a light source is originally apolarization laser, and the polarization beam splitter 9 is installed soas to reflect all the laser beams.

Meanwhile, the pattern to be inspected, which has been formed on thewafer 1 through semiconductor processes, exhibits various shapes.Therefore, a reflecting beam from the pattern has various polarizationcomponents. The polarization devices 10 control the polarizationdirection of the laser lighting beam and reflecting beam so as to have afunction to arbitrarily adjust a polarization ratio of the lightingbeam. The function prevents uneven brightness of the reflecting beamcaused by pattern shapes or density difference from reaching the imagesensor 13. The polarization devices comprise, for example, a ½wavelength plate and a ¼ wavelength plate.

The image sensor 13 is, for example, a time delayed integration sensor(TDI sensor), which outputs shading image signals in response to thebrightness (thick or thin) of the reflecting beam from the semiconductorwafer 1 having one exemplary pattern to be inspected. An A/D converter14 converts the shading image signals 13 a obtained from the imagesensor 13 to digital signals. In other words, the stage 2 is scannedwhile the semiconductor wafer 1 having one exemplary pattern to beinspected is moved at a constant speed, so that a focus detection system(not shown) always detects the position of the surface to be inspectedin the direction of the Z axis. The stage 2 is thereby controlled in thedirection of the Z axis so that the space between the objective lens 11and the surface to be inspected is kept constant. Then, the image sensor13 detects brightness information (shading image signals) of the patternformed on the semiconductor wafer with high accuracy.

The reference numeral 15 represents, for example, an 8-bit typegradation converter, which conducts logarithmic, exponential, andpolynomial transformations on digital image signals outputted from theA/D converter 14 so as to correct uneven image brightness caused byinterference between the laser beam and a thin film formed on thesemiconductor wafer 1 during the processes. A delay memory 16 stores anddelays output image signals from the gradation converter 15 for onecell, one chip or one shot constituting the semiconductor wafer 1 with ascanning width of the image sensor 13. A comparator 17 compares imagesignals outputted from the gradation converter 15 with image signalsobtained by the delay memory 16 to detect disparities as defects. Thecomparator 17 compares the detected image with the image that isoutputted from the delay memory 16 and delayed with an amountcorresponding to a cell pitch, etc. Coordinates such as arrangement dataon a semiconductor wafer 1 obtained based on design information areinputted with an input means 18 including a keyboard, a disk, etc., andthereby a CPU 19 creates and stores in a storage device 20 defectinspection data based on coordinates such as arrangement data on thesemiconductor wafer 1, whose comparison results by the comparator 17have been inputted.

The defect inspection data, if necessary, can be displayed on a displaymeans 21 such as a display, and further outputted to an output means 22so that defect points can be observed, for example, with other reviewdevices. The comparator 17 comprises, for example, a circuit forpositioning images, a differential image detection circuit of positionedimages, a disparity detection circuit for digitalizing differentialimages, and a feature extraction circuit for extracting areas andlengths, coordinates, and other factors from the digitalized outputs.

In addition, the configuration of the low coherent optical unit 6 of thelighting optical machine is described. In general, the laser hascoherence properties, and laser-lighting on a wafer may be a cause forgenerating speckle noise from a circuit pattern. Thus, in the case oflaser-lighting, it is necessary to reduce coherence.

Either of temporal or spatial coherence can be reduced for reducing thecoherence. In the present invention, a laser beam is two-dimensionallyscanned by two scanning mirror mechanisms 71 and 74, which areapproximately orthogonal to each other and whose reflecting facesrevolve in the direction indicated by the arrow as shown in FIG. 3, toreduce spatial coherence.

The low coherent optical unit 6 is described in detail by furtherreferring to FIG. 2. The laser beam L1 is emitted from the laser beamsource 3 and expanded to a certain size by the beam expander 5 to becomea parallel pencil, which is reflected by the scanning mirror mechanism71, converged with the lens 72, and then made to become a parallelpencil again with the lens 73. After the parallel pencil is reflected bythe scanning mirror mechanism 74, it is converged on the center 11 a ofthe objective lens with the lens 7. The mirrors of the scanning mirrormechanisms 71 and 74 are in conjugate positions relative to the surfaceof the wafer 1. The scanning mirror mechanisms 71 and 74 haveoscillating mirrors that revolve or oscillate with electric signals, andthe laser beam L1 is thereby two-dimensionally scanned on the pupil ofthe objective lens 11. Examples of the electric signals to be inputtedinto the scanning mirror mechanisms 71 and 74 include triangular wavesand sinusoidal waves, and variations of the frequency or amplitude ofthe inputted electric signals enables the scanning of various shapes onthe pupil 11 a of the objective lens 11.

As described above, the beam polarization mechanism and beam expanderadjustment mechanism are provided relative to the laser lighting opticalsystem. Since the lighting optical mechanism is stored in the housing,an operator can adjust the beam without directly touching the opticalsystem and there is no fear that the beam would leak to the outside,resulting in safe operations. This allows the operator to be free fromoperations in a narrow space, so that safer operations are assured.

In addition, a beam spot positioning sensor or a camera for observingbeam profile is provided to automatically control the beam polarizationmechanism and beam expander adjustment mechanism, so that a stable beamcan constantly be applied to an object to be measured.

EFFECT OF THE INVENTION

The present invention provides a lighting optical machine and a defectinspection system that are highly reliable and safe when a laser beam isused as a light source.

All publications, patents and patent applications cited herein areincorporated herein by reference in their entirety.

1-6. (canceled)
 7. A defect inspection method of a defect inspectionapparatus including a housing, wherein the housing accommodates a laserbeam source, a beam deflection mechanism enabling a beam emitted fromthe laser beam source to be reflected so that the beam travels in adirection almost parallel to the beam emitted form the laser source, abeam expander for converting the beam to a parallel beam having a largercross-sectional area, an objective lens, through which the parallel beamis reduced and applied to the surface of a sample and a convergencelens, the method comprising the steps of: splitting the parallel beam ina light passage from the beam expander to the objective lens into atleast two split parallel beams; further dividing at least one of thesplit parallel beams into at least two divided parallel beams; observinga beam intensity profile of the cross-section of a first of the dividedparallel beams; converging a second of the divided parallel beams withthe convergence lens; detecting a position of a spot image convergedwith the convergence lens; and, controlling the beam deflectionmechanism and the beam expander on the basis of either or both ofinformation of the cross-sectional diameter of the beam and informationof the beam position displacement.
 8. The defect inspection method asset forth in claim 7, further comprising the steps of: forming anenlarged image of the sample irradiated with a second divided parallelbeam; and, comparing images of two areas on the sample to detect adefect.